Each photodetector of an array may be required to be responsive to a different wavelength signal. An incoming composite signal may comprise a number of different wavelength signals and have been transmitted down an optical fibre, for example such as in Optical transmission systems e.g. FDM (frequency division multiplexing) applications. The composite signal may be suitably split up, such as by a grating, whereby to address the respective photodetector of an array with the appropriate part of the composite signal. When several such optical signals are being received simultaneously by an array of photodetectors, it is sometimes required to process only one such signal at a time. In this case only a single amplifying channel is required behind the photodetectors. Normal PIN detectors cannot be used in such arrays with a single amplifying channel since they give an electrical output even when no voltage is applied across them, it being well known that a PIN detector will produce a photovoltaic signal under zero bias.
An array of photoconductors as photodetectors can be used successfully with a single amplifying channel since a photoconductor will only just give an output when a voltage is applied. There are, however, practical problems associated with photoconductors. Photoconductors for use at 1.3 to 1.5 um wavelengths are not well developed and may present problems in their frequency response. The high purity of the InGaAs which is required for the photoconductor is close to the best achievable at present.
It is thus an object of the present invention to provide an alternative form of optical detector.